This is from another post, but the exact same problem has happened on my 1996 V8 Explorer.
http://img373.imageshack.us/img373/1112/img1210kz1.jpg
http://img520.imageshack.us/img520/4348/img1213kj1.jpg
Apparently, my dipstick ring broke or fell off while driving the car, and the dipstick...